This invention generally relates to continuous sensors used in steel manufacturing and other metallurgical processes. Such sensors include slag detectors, thermocouples, and sulfur and oxygen probes that generate a DC output, and the invention is particularly concerned with the use of an impedance monitoring circuit in combination with such probes for determining the reliability of the probe signals.
Probes for monitoring various conditions in metallurgical processes such as steel making are known in the prior art. For example, a slag detecting sensor is known that detects the presence or absence of slag in a flow of molten metal by a coil of wire which generates a fluctuating magnetic field that interacts with the flow. More recently, a slag detecting sensor has been developed that comprises an insulated conductive pin that comes into direct contact with a flow of molten metal through a ladle shroud. Such a probe is disclosed and claimed in U.S. Pat. No. 5,375,816. The contact between the liquid metal and the electrically conductive pin creates a very small potential due to thermocouple effects. This DC voltage substantially increases upon the introduction of slag into the flow of molten metal due to the presence of charged particles in slag, which is believed to create an electrical double layer around the conductive pin. The electrical double layer in turn induces a substantial increase in the DC potential relative to ground of a few hundred millivolts. When such a pin is connected to a grounded volt meter, the resulting circuit has been found to provide a simple and reliable means for immediately detecting the presence of slag in a flow of molten metal such as steel.
In addition to slag detectors, thermocouple probes are often employed in such processes for continuously monitoring the temperature of the molten metals at various locations in the manufacturing facility. The sensor circuits of such thermocouple probes generally comprise a junction of dissimilar metals, such as platinum and rhodium, which generate low DC voltages when exposed to a sufficiently high temperature. Other types of probes used in metallurgical processes include sulfur and oxygen sensors for detecting the concentration of dissolved sulfur and oxygen in molten steels. Such probes are formed from a solid electrolyte encased in a heat-resistant shield. The shield protects the electrolyte from thermal shock, and ablates when the probe is immersed in molten metal. In operation, the conduction of the ions of sulfur or oxygen through the electrolyte generates a low voltage DC potential across the electrolyte whose magnitude is related to the solute activity in the molten metal.
While each of the aforementioned probes has demonstrated its utility in monitoring the particular condition it is designed for, problems can arise when the components in these probes either fail or begin to approach a break-down condition due to either mechanical or thermal shock, or a subtle electrical fault, such as a breaking (but not yet broken) conductor. The applicant has observed that such probes are capable of generating a low-voltage signal even in a failed or near failure condition. For example, probes such as the oxygen and sulfur sensors can still generate a voltage due to stray electromagnetic fields if their internal contacts should become broken. In instances where such a spurious voltage is within the range associated with the normal operation of the metallurgical process, they are dangerously misleading, as they provide a signal which may be completely false. Accordingly, there is a need for a way to determine whether or not an apparently normally functioning probe is in a failed or near failure condition.
In solving the aforementioned problem, the applicant has observed that the resistance of a probe in a failed or near failure condition changes to an extent to where the probe circuit resembles either a closed circuit (which may happen in the event of a short circuit) or an open circuit (which may happen in the event of a broken lead wire). Hence it is possible to test the reliability of the voltage signal generated by a probe by means of a simple DC resistance meter. However, such a solution has two major drawbacks. First, the imposition of a calibration voltage across a probe circuit alters the output signal of the probe, which in turn renders this approach incompatible with continuous monitoring. Consequently, such an approach would not be able to detect with any precision the exact moment of a failed or near failure condition, as the probe could only be intermittently monitored. Secondly, even the imposition of an intermittent DC voltage across the circuits of many of the aforementioned probes could result in unwanted polarization effects that could substantially distort the resulting output signals either temporarily or permanently. For example, if a calibration voltage were applied across the electrolyte present within a sulfur or oxygen sensor, the resulting potential could cause a maximum migration of the ions in the electrolyte, which could permanently ruin the reliability of any output signal subsequently generated by such a probe.
Clearly, there is a need for a way of determining whether or not the circuit of a particular metallurgical probe is in a failed or near failure condition which could continuously monitor the condition of the probe without altering its voltage signal output. Ideally, such a technique would not involve the application of any DC voltages to the components of the probe circuit which could result in unwanted polarizations in the interface between the probe sensor and the material being sensed. Finally, such a technique should be easy and inexpensive to implement, and readily applicable to the circuits of probes that are already in service.